One conventional system for controlling the temperature and humidity of the air introduced into the work space in an air conditioned plant consists of controlling the conditioning of the air in a central unit by first reducing the temperature of the air to a temperature corresponding to the dewpoint temperature desired in the work space, thus saturating the air with moisture at that temperature, and thereafter adding sensible heat to the air so as to elevate the temperature to the dry bulb temperature desired in the space. While such a method facilitates the control of the humidity of the air entering the conditioned space from the central unit, it uses unnecessary energy when it is necessary to first cool the air to a temperature corresponding to the dewpoint temperature and thereafter reheat the air.
In order to conserve energy, the control shown in U.S. Pat. No. 4,399,864 provides an improved control system for a central unit in which the air introduced into the work space is a controlled mixture of fresh air and secondary air which is subjected to a controlled humidification prior to introduction into the work space. The mixture of secondary air and fresh air is controlled in conjunction with the washer spray to insure that the air is of a sufficiently low humidity to provide the desired humidity in the work space. Sensible heat is provided to the mixture, when needed, either by heating the secondary air prior to its mixture with primary air or by heating the mixture. The control is effected by sensing one property or condition of the introduced air, e.g. the wet bulb temperature, and modulating the controls to effect the necessary changes in that property to maintain the desired temperature and humidity in the room.
The control of the room conditions in this manner is generally satisfactory, but when the ambient conditions vary to a large degree, such control tends to be inefficient and wasteful of energy.
The changes in the ambient conditions include variations in the amount of sensible heat which is added to the air in the work space to be conditioned, which may occur when a large number of heat-producing machines are started or stopped, when the lighting load is substantially altered, or when the heat-generating components in the air conditioned space are shut down, for example between operating shifts. The ambient conditions may also be affected by changes in the outdoor climate. Wide variations in the outdoor air may require substantial modulation of the central air unit in order to accommodate to the changes in the outdoor air conditions. Furthermore, after periods of use, different washer systems in the central unit may operate with different degrees of efficiency and cause a cumulative variation in the effectiveness of a particular unit.